Method of evaluating the capillary pressure curve of an underground deposit rocks based on rock cuttings measurements

ABSTRACT

A method of evaluating the capillary pressure curve of rocks of an underground reservoir from measurements on rock debris or fragments such as cuttings from the reservoir, over the total saturation range of these rocks, within a short period and at a low cost, from these measurements is disclosed. The method comprises measuring the permeability k of the debris, measuring the capillary pressure curve Pc as a function of the saturation of these fragments initially saturated with a fluid (brine for example) by subjecting them to centrifugation, and parametrizing a capillary pressure curve Pc satisfying empirical relations depending on adjustable parameters, constrained to adjust to an asymptotic part of the capillary curve measured by centrifugation, and to the value of permeability k measured on the cuttings, so as to obtain the whole of the capillary pressure curve. Applications include hydrocarbon reservoir evaluation.

FIELD OF THE INVENTION

The present invention relates to a method of evaluating a capillarypressure curve of the rocks of an underground reservoir frommeasurements on rock debris taken therefrom.

BACKGROUND OF THE INVENTION

Laboratory Measurements on Cores or Cuttings

Measurement of petrophysical parameters such as the permeability, theporosity and the capillary properties on rock fragments obtained whiledrilling a well through an underground formation is an interestingopportunity for operator companies to rapidly obtain a firstpetrophysical characterization of producing zones traversed by the well.

Patent FR-2,809,821 filed by the applicant describes a system ofevaluating physical parameters such as the absolute permeability ofporous rocks of an underground reservoir zone, from cuttings returned tothe surface in the drilling mud. In an enclosure where the cuttings aredipped in a viscous fluid, some of this fluid is injected at a pressurethat increases with time until a predetermined pressure threshold isreached, so as to compress the gas trapped in the pores of the rock.This injection stage is followed by a relaxation stage where injectionis stopped. The pressure evolution during the injection process beingmodelled from initial values selected for the physical parameters of thecuttings, a computer adjusts them iteratively so as to obtain the bestpossible agreement between the modelled pressure curve and the pressurecurve really measured.

Patent application FR-02/0023 filed by the applicant describes anothermethod of evaluating physical parameters such as the absolutepermeability and the porosity of the rocks of an underground reservoirzone, also from cuttings. An enclosure containing the rock fragments andfilled with a viscous fluid is communicated with a vessel containing thesame fluid at a predetermined pressure so as to compress the gas trappedin the pores of the rock. The time of application of this pressure,according to whether it is short or long, allows to measure either thepressure variation in the enclosure or the variation of the volumeactually absorbed by the rock fragments. The pressure or volumeevolution in the enclosure is then modelled from initial values selectedfor the physical parameters of the fragments, and the values of thephysical parameters of the rock fragments are iteratively adjusted sothat the modelled evolution best adjusts to the measured evolution ofthe physical parameter in the enclosure.

In the sphere of petrophysical characterization, the capillary pressureis also a very important datum for operators because it conditions:

-   -   the initial distribution of the fluids in the reservoir from the        aquifer zone (referred to as WOC, for water-oil contact, by the        man skilled in the art) to the upper part of the reservoir        (transition zone). According to the capillary pressure curve        associated with a reservoir rock and the nature of the fluids in        place, this transition zone can extend over some meters or some        ten meters, which has an important effect on the determination        of the accumulations in place,    -   the input pressure of a rock, which is particularly important        for cap rocks. For example, for a gas storage tank, the input        pressure of the cap rocks conditions directly the allowable        overpressure in the storage levels without having leaks.

With the current techniques, the capillary pressure curve is obtained bymeans of laboratory measurements on reservoir cores. These methods areexpensive because of the coring operations as well as the measurementson the cores, and the results are often available only several monthsafter drilling.

Approaches to Rapidly Obtain the Capillary Pressure Curve

However, there are alternative methods described in the literature forevaluating the capillary pressure curve rapidly, either during drillingor slightly later.

The most commonly used approach consists in using the mercuryporosimetry technique for measuring the air/mercury capillary pressurecurve Pc directly from cuttings. However, the curve obtained issignificantly different from the reference curve obtained on cores withhigh wetting fluid saturations. Besides, this approach is based on theuse of mercury, which is extremely polluting and progressively forbiddenby the law in many countries, which poses a major problem for applyingthis technique in the near future.

Another known method uses the Nuclear Magnetic Resonance (NMR) techniqueto rapidly estimate the capillary pressure curve from stratigraphic datameasured in the well shortly after drilling. It is notably described inthe following publications:

-   -   Bowers, M., A. et al.: <<Prediction of permeability from        capillary pressure curves derived with NMR>>, 17 Sep. 1998,    -   Marshall, D., et al.: <<Method for correlating NMR relaxometry        and mercury injection data>>, SCA No. Society of Core Analysts        International Symposium 1995,    -   Volokitin, Y., W. J. et al.: <<A practical approach to obtain        1^(st) drainage capillary pressure curves from NMR core and log        data>>, SCA No. Society of Core Analysts International Symposium        1999.

The NMR relaxation signal is first converted in terms of pore sizedistribution, then in terms of threshold size distribution, which allowsto calculate a pseudo-capillary pressure curve. This approach has beentested on several samples of known curve Pc. The results show that agood agreement with the reference curves can only be obtained by meansof a rigorous calibration stage to be carried out case by case accordingto the nature of the rocks studied. This calibration stage is necessaryowing to the uncertainty on:

-   -   the NMR signal-pore size distribution conversion which depends        on the value of the surface relaxivity which is variable        according to the rocks, and    -   the pore size distribution-threshold size distribution        conversion which depends on the nature of the rock and on the        diagenesis process.

This approach is therefore not recommended in a predictive explorationcontext. In any case, it would not be applicable to cuttings.

Image analysis has also been the subject of work intended to obtain acurve Pc. The porous medium is first prepared in form of a thin sectionphotographed by scanning electron microscopy or SEM. The image obtainedis then analysed so as to determine parameters representative of theproportion and of the shape of the voids in relation to the rock. Inparticular, it is possible to determine a threshold size distribution toreconstruct a pseudo-capillary pressure curve Pc. The main limitation ofthis method is the two-dimensional (2D) nature of the thin section,whereas the capillary pressure is by definition a three-dimensional (3D)property. Besides, this technique requires quite heavy conditioning,which is not really compatible with a result obtained slightly later.Image analysis could be applicable on cuttings but it would requirecareful calibration to acquire a good predictability.

Finally, it can be noted that the centrifuging technique is sometimesapplied in the field to cuttings, but in order to extract the largestpossible amount of drilling fluid from the rock to minimize pollutantdischarges to the environment and to limit the cost by recycling thedrilling fluid recovered. As far as we know, no centrifugation oncuttings has been considered in order to determine capillary properties.

SUMMARY OF THE INVENTION

The method according to the invention allows to determine the capillarypressure curve of rocks of an underground reservoir from measurements onrock debris or fragments (such as cuttings) taken therefrom, over thetotal saturation range of these rocks, within a short period and at alow cost, from these measurements. It comprises:

-   -   measuring the permeability k of the rock debris,    -   measuring the capillary pressure curve Pc as a function of the        saturation of the rock debris initially saturated with a fluid        by subjecting them to centrifugation, and    -   parametrizing a capillary pressure curve Pc satisfying empirical        relations depending on adjustable parameters, that is        constrained to adjust to an asymptotic part of the capillary        curve measured by centrifugation, and to the value of        permeability k measured on the cuttings, so as to obtain the        whole of the capillary pressure curve.

Parametrizing the curve is advantageously carried out by selecting bydefault a set of said parameters allowing calibration on the asymptoticpart of the capillary pressure Pc with low saturations, and by modifyingthe parameters step by step so that the estimation of the permeabilitygiven by one of the empirical relations used is best adjusted with themeasurements of permeability k carried out on rock debris and with thisasymptotic part.

Permeability k of the cuttings is measured for example from measurementsof the pressure variations in a vessel filled with a fluid containingthe cuttings after it has been communicated for a predetermined periodof time with a tank containing the same fluid under pressure, and fromthe volume actually absorbed by the cuttings, and from modelling theevolution of the pressure or of the volume in the vessel, from initialvalues selected for the physical parameters of the cuttings, which areiteratively adjusted so that the modelled pressure evolution bestadjusts with the measured evolution of the physical parameters of thecuttings.

The method is notably advantageous in that it provides the capillarypressure of the rocks on the basis of simple cuttings that are morereadily available and less expensive to obtain. The results are alsoobtained much more rapidly than with cores.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the method according to the inventionwill be clear from reading the description hereafter of an embodimentgiven by way of non limitative example, with reference to theaccompanying drawings wherein:

FIG. 1 shows the evolution of the NMR signal during variouscentrifugation stages carried out on cuttings,

FIG. 2 a shows the curves Pc obtained on a single rock (B7). fromcuttings and from a core,

FIG. 2 b shows the comparison between the reference curve Pc measured ona core and the reconstructed curve Pc over the total saturation rangefrom the centrifugation measurements and the permeability measurement,

FIGS. 3 a and 3 b show results comparable to those shown in FIGS. 2 aand 2 b respectively for another rock GDV1,

FIGS. 4 a and 4 b show results comparable to those shown in FIGS. 2 aand 2 b respectively for another rock Rot1,

FIGS. 5 a and 5 b show results comparable to those shown in FIGS. 2 aand 2 b respectively for another rock St Max,

FIG. 6 shows the comparison between the reference permeabilities and thepermeabilities measured on cuttings within the scope of the methoddescribed in the aforementioned patent application FR-02/0023, and

FIG. 7 diagrammatically shows the flowchart for implementing the method.

DETAILED DESCRIPTION

The method intended for fast evaluation of a capillary pressure curve Pcfrom rock fragments or cuttings according to the invention isillustrated by FIG. 7. The method is based on two experimental measuringstages, followed by a parametrizing stage by reference with knowncurves, to reconstruct curve Pc over the total saturation range. Duringone of the measuring stages, capillary pressure data Pc are acquired bycentrifuging the initially water-saturated rock fragments. The otherexperimental stage allows to calculate the permeability value k of therock from the method described in the aforementioned patent applicationFR-02/0023. Reconstruction of curve Pc over the total saturation rangeis carried out by using a parametrized form. The parameters of the curveare determined in such a way that the curve matches the capillarypressure data Pc obtained experimentally and the estimated permeabilityvalue from the curve Pc obtained according to a known method such asThomeer's or Swanson-Kamath's method, which are reminded below, with thevalue measured on the rock fragments. Several application cases arepresented, with show the very good agreement obtained with referencecurves without any particular previous calibration procedure.

I) Measurement of Pc by Centrifugation From the Cuttings

To implement the method, it is possible to use standard centrifugationmeans or more sophisticated means with automatic monitoring of thevolumes of fluid produced, such as those described for example inpatents EP-603,040 (U.S. Pat. No. 5,463,894), FR-2,763,690, FR-2,772,477(U.S. Pat. No. 6,185,985) or FR-2,798,734 filed by the applicant.

The cuttings which have come up to the surface during the drillingoperation are first cleaned with solvents in a Soxhlet type device, thendried and saturated with 30 g/l brine. The cuttings are then drained ina damp cloth so as to remove the water trapped between the variouscuttings d, then placed in a cell or cup fastened to the end of arotating arm. The water expelled by centrifugation from the cuttingsflows through a grate and it is collected at the base of the cup. Theexperimental data are acquired in the same way as in the context ofcentrifugation on a core. For a centrifugation stage (given rotatingspeed), the evolution of the water production is measured until nosignificant variation can be observed any longer, then the rotatingspeed is increased to start a new stage.

As can be seen in FIG. 1, a progressive decrease of the NMR signal and ashift to the short relaxation times T2 are observed, which express adesaturation of the porous medium with the increase of the rotatingspeed. This type of measurement shows that there definitely is acapillary contact between the cuttings, which allows to measure Pc bycentrifugation from cuttings.

The volume of water produced during the experiment is converted tosaturation data from the volume of water initially contained in thecuttings. The latter is determined by weighing (difference in the weightof the cuttings before and after saturation) or directly by NMRmeasurement.

FIGS. 2 a to 5 a show the result of experiments carried out from modelcuttings, 1 to 2 mm in size, manufactured in the laboratory from rocksof known properties for which a curve Pc conventionally measured bycentrifugation of a core is available. It can be seen that a goodagreement is obtained with the reference curve at the level of theasymptotic part (low wetting fluid saturation). On the other hand, a bigdifference is observed for higher wetting fluid saturations. Resultsequivalent to those obtained within the context of porosimetrymeasurements using mercury on cuttings are thus obtained, withoutpollution risks.

The measured capillary pressure curve Pc is however representative onlyon the asymptotic part. A reconstruction procedure is thereforenecessary to evaluate the behaviour of the curve over the totalsaturation range.

II) Measurement of the Permeability k of the Cuttings

The method described in the aforementioned patent application 02/02,242is applied to measure the permeability of the cuttings. The cuttings aretherefore dipped in a containment enclosure containing a viscous fluid.The enclosure is then communicated with a vessel containing the samefluid under pressure, so as to compress the gas trapped in the pores ofthe rock. According to a first embodiment, this communication period canbe very short and followed, after a latency time, by the measurement ofthe pressure evolution in the enclosure. According to anotherembodiment, the communication period can be long enough to allow toobserve and measure the variation of the volume actually absorbed by thecuttings.

The evolution of the pressure or of the volume in the enclosure is thenmodelled from initial values selected for the physical parameters of thecuttings, and the values of the physical parameters of the cuttings areiteratively adjusted so that the modelled evolution best adjusts withthe measured evolution of the physical parameter in the enclosure.

This procedure gives excellent results. The permeability values k of thecuttings are totally in accordance with the reference measurementsobtained from cores.

III) Reconstruction of the Total Capillary Pressure Curve

In this third stage, we synthesize the previous measurements that haveallowed to construct the asymptotic part of the capillary pressure curvePc and the measurements of permeability k using empirical relationsreputed to model the physical parameters of rocks well. The followingpublications:

-   -   Thomeer, J. H. M.: “Introduction of a pore geometrical factor        defined by the capillary pressure curve”, Trans AIME, vol.        March, pp. 73-77, 1960, and    -   Thomeer, J. H. M.: “Air permeability as a function of three pore        network parameters”, Trans AIME, vol. April, pp. 809-814, 1983        describe methods for evaluating the permeability from a        capillary pressure curve. Capillary pressure curve Pc is        modelled in the following form:        $P_{c} = {P_{d} \times {\exp\left( {- \frac{G}{{Ln}\left( \frac{V_{b}\left( P_{c} \right)}{V_{b}\left( P_{\infty} \right)} \right)}} \right)}}$        where:    -   G is a shape parameter for taking account of the curvature of        the capillary pressure curve (related to the shape of the pore        size distribution),    -   P_(d) the displacement pressure extrapolated to S_(Hg) equal to        zero, and    -   V_(b) _(oo) the percentage of volume occupied by the mercury at        the end of the experiment at an infinite capillary pressure        (equal to φ×S_(Hg)),

The three parameters of the model being related to the permeability bythe following expression:$k = {3.8068 \times G^{- 1.334} \times {\left\lbrack \frac{V_{b\infty}}{P_{d}} \right\rbrack^{2}.}}$

In the following publication:

-   -   Swanson, B. F.: “A simple correlation between permeability and        mercury capillary pressures”, JPT, vol. December, pp. 2498-2504,        1981,        the author proposes correlating the value of the permeability        with the maximum value of ratio (V_(b)/P_(c)) on the mercury        porosimetry curve. This particular point generally corresponds        to the regime change that occurs at the end of the percolation        regime, just before the significant capillary pressure increase.        As regards the capacity of fluid flow in the porous medium, this        point is particularly important because it represents the pore        size for which the entire pore network is connected and which        therefore controls the flow. The most general correlation        provided by the author is given by the expression (Swanson        1981):        $k = {355 \times {\left( \frac{V_{b}}{P_{c}} \right)_{A}^{2.005}.}}$

The following publication:

-   -   Kamath, J.: “Evaluation of accuracy of estimating air        permeability from mercury injection data”, SPE Formation        evaluation, vol. 7,4, pp. 304-310, 1992        also relates to the comparative evaluation of the approaches        using empirical correlation or physical models to determine the        permeability value from a mercury porosimetry curve. The best        agreement is obtained with a new correlation based on the        aforementioned characteristic length by Swanson 1981, mentioned        above: k = 413 × L_(max)^(1.85)  if  k < 1  m  D        k = 347 × L_(max)^(1.60)  if  k > 1  m  D,        L_(max) being defined as follows:        $L_{\max} = {\left( \frac{\phi \times S_{nw}}{P_{c}} \right)_{\max} = {{\frac{\phi \times \lambda \times \left( {100 - S_{r}} \right)}{P_{e} \times \left( {1 + \lambda} \right)^{\frac{1}{\lambda} + 1}}\quad{and}\quad\left( \frac{P_{e}}{P_{c}} \right)^{\lambda}} = \frac{S_{w} - S_{r}}{100 - S_{r}}}}$        where:    -   λ: exponent expressing the curvature of the capillary pressure        curve (related to the shape of the pore size distribution),    -   P_(e): the displacement pressure extrapolated to S_(Hg) equal to        zero, and    -   S_(r): residual saturation occupied by the wetting fluid (%).

To parametrize the empirical capillary pressure curve Pc obtained fromThomeer's, Swanson's or Kamath's approaches, it is constrained to adjustto the asymptotic part obtained by centrifugation during the first stageof the method. The entire curve is constrained using also the value ofpermeability k measured on cuttings during the second stage of themethod, which is compared with the result of the empirical relations.The parameters of the capillary pressure Pc are then modified until boththe measured asymptotic behaviour and the permeability estimation aremet, which allows to constrain the capillary pressure curve over thetotal saturation range Sw.

The inversion process starts with a set of default parameters whichallow to calibrate the asymptotic behaviour of the capillary pressure Pcwith low water saturations. These parameters are then modified step bystep (mainly the input pressure Pe or Pd and the shape factor λ or G) sothat the estimation of the permeability given by one of the previousrelations is in good agreement with the permeability measurementobtained on cuttings while keeping a good agreement with themeasurements of Pc at low water saturations.

1.1 Results obtained

FIGS. 2 b, 3 b, 4 b and 5 b show the comparison between the curve Pcreconstructed according to the previous procedure and the referencecurve Pc obtained on a core. It can be seen that, whatever the exampleconsidered, the reconstruction method allows to obtain a pertinentevolution of Pc over the total saturation range and in particular athigh water saturations, whatever the permeability of the rock. K Vb PeName (mD) G (fraction) (bar) Rot1 150 0.28 0.89 0.38 GDV1 195 0.28 1.000.41 B7 780 0.25 0.92 0.2 StMax 2000 0.34 0.92 0.26

1-3. (canceled)
 4. A method of determining a capillary pressure curve ofrocks of an underground reservoir from measurements on cuttings takentherefrom, by measuring a permeability of cuttings, comprising:measuring the capillary pressure curve as a function of saturation bysubjecting the cuttings, initially saturated with a fluid, tocentrifugation; and parametrizing the capillary pressure curvesatisfying empirical relations depending on adjustable parameters,constrained to an asymptotic part of the capillary curve measured bycentrifugation, and to a value of the permeability measured on thecuttings, so as to obtain a capillary pressure curve.
 5. A method asclaimed in claim 4, comprising: selecting by default a set of theparameters allowing calibration on an asymptotic part of the capillarypressure curve and modifying parameters so that an estimation of thepermeability given by one of the empirical relations is adjusted withpermeability measurements carried out on the cuttings with theasymptotic part.
 6. A method as claimed in claim 4, wherein thepermeability of the cuttings is measured from measurements of pressurevariations in a vessel filled with a fluid containing the cuttings afterthe vessel has been coupled for a predetermined period of time to asource of fluid containing the cuttings under pressure, a volumeabsorbed by the cuttings, and modelling an evolution of the pressure orof the volume of the vessel, from initial values selected for a physicalparameters of the cuttings, which are iteratively adjusted so that amodelled pressure evolution adjusts with a measured evolution of thephysical parameters of the cuttings.
 7. A method in accordance withclaim 4 wherein: the asymptotic part has low saturation.
 8. A method inaccordance with claim 5 wherein: the asymptotic part has low saturation.9. A method in accordance with claim 6 wherein: the asymptotic part haslow saturation.
 10. A method in accordance with claim 4 wherein themodelled pressure evolution best adjusts with a measured evolution ofphysical parameters of the cuttings.
 11. A method in accordance withclaim 5 wherein the modelled pressure evolution best adjusts with ameasured evolution of physical parameters of the cuttings.
 12. A methodin accordance with claim 6 wherein the modelled pressure evolution bestadjusts with a measured evolution of physical parameters of thecuttings.
 13. A method in accordance with claim 7 wherein the modelledpressure evolution best adjusts with a measured evolution of physicalparameters of the cuttings.
 14. A method in accordance with claim 8wherein the modelled pressure evolution best adjusts with a measuredevolution of physical parameters of the cuttings.
 15. A method inaccordance with claim 9 wherein the modelled pressure evolution bestadjusts with a measured evolution of the physical parameters of thecuttings.
 16. A method in accordance with claim 4 wherein a wholecapillary pressure curve is obtained.
 17. A method in accordance withclaim 5 wherein a whole capillary pressure curve is obtained.
 18. Amethod in accordance with claim 6 wherein the source of fluid is a tank.19. A method in accordance with claim 7 wherein the source of fluid is atank.
 20. A method in accordance with claim 8 wherein the source offluid is a tank.
 21. A method in accordance with claim 9 wherein thesource of fluid is a tank.
 22. A method in accordance with claim 10wherein the source of fluid is a tank.
 23. A method in accordance withclaim 11 wherein the source of fluid is a tank.